Fungi are very important ecologically, because they are one of the most important decomposers in most ecosystems. They help to break down plant fibers such as dead leaves and wood and allow the nutrients to be released into the soil.

Fungi are useful to humans because they have economic value. Some fungi, such as mushrooms and truffles, are eaten directly, and others are used to produce things of value. Several important antibiotics, including penicillin, were discovered by studying the fungi that naturally produce them.

On the negative side, there are many fungi that can cause infections of varying severity in humans. Athlete's foot, ringworm, and thrush are all fungal infections of the surface of the body. A variety of fungi can also cause respiratory infections.

Fungi also cause wastage and loss of foods. Some fungi, such as powdery mildew and ergot, can attack and spoil crops before they are harvested, while many others cause food spoilage when food is being transported to market or is in storage.